Check Engine light. Misfire Cylinder 4. Keeps coming back

Had a Check Engine light. Misfire Cylinder 4. Code was P0304. I changed the plugs, wire, coil pack. Everything was fine check engine light went away for 24 hours. After the 24 hours the light came back. Had codes ran again and the same code came back. I then replaced the fuel injectors and the code and light went away. I had the computer scanned and came up no code/no errors found. Now today 2 days later, the check engine light is back. I figured it just might be a build up so I went to the store to get some seafoam however they were sold out so I used some lucas fuel management and Unhooked the battery to reset the check engine light.
Anyone know what this could be? Any tips advice? Anything?
Im getting terrible gas mileage. I was told that I need to get the codes scanned and get the computer reset? Anyone know how to reset a computer???